lxr.php.net has been around for a while, but since I've found myself using it a lot recently, I want to point it out. It's such a quick way to find the particular file a built-in (or extension) PHP function is defined. Coupled with the revision graph that cvs.php.net can display, it is handy for tracking down exactly when a function was added or a bugfix was committed.

The site, while a bit dated, is still very useful as a cross-referencing tool to research the PHP source code.